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Lecture Notes 9

Lecture Notes 9 - 9 Chemical Bonding I Basic Concepts bond...

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9. Chemical Bonding I: Basic Concepts bond formation: valence electrons transfer of electrons from one atom to another: cations and an- ions = ionic bond strength of ionic bond Coulomb potential energy E Q cat · Q an r r = distance between cation and anion, Q = charge E is negative: formation of ionic bond is exother mic the more negative E , the stronger the bond the larger the charge, the more negative E the smaller the ion, i.e., the smaller r , the more negative E lattice energy = quantitative measure of ionic bond strength lattice energy = energy required to completely separate one mole of a sol id ionic compound into gaseous ions GChem I 9.1
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the more positive the lattice energy, the stronger the bond lattice energy is determined indirectly via the Born-Haber cycle , which is based on Hess’s law and relates the lattice energy of an ionic compound to ionization energies, electron affinities, and other atomic and molecular properties example: lattice energy of lithium fluoride standard enthalpy of formation of lithium fluoride: Li(s) + 1 2 F 2 (g) -→ LiF(s) Δ H f =- 594.1kJ/mol The formation reaction can be carried out in five steps instead: (1) Convert solid lithium to lithium vapor (sublimation): Li(s) Li(g) Δ H 1 = 155.2kJ/mol (2) Dissociate 1 2 mole of F 2 gas into gaseous F atoms: 1 2 F 2 (g) F(g) Δ H 2 = 75.3kJ/mol (3) Ionize one mole of gaseous Li atoms: Li(g) Li + (g) + e - Δ H 3 = 520kJ/mol GChem I 9.2
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(4) Add one mole of electrons to one mole of gaseous F atoms: F(g) + e - -→ F - (g) Δ H 4 =- 328kJ/mol (5) Combine one mole of gaseous Li + and one mole of gaseous F - to form one mole of solid LiF : Li + (g) + F - (g) LiF(s) Δ H 5 = ?kJ/mol Note : lattice energy = - Δ H 5 Li(s) Li(g) Δ H 1 = 155.2kJ/mol 1 2 F 2 (g) F(g) Δ H 2 = 75.3kJ/mol Li(g) Li + (g) + e - Δ H 3 = 520kJ/mol F(g) + e - F - (g) Δ H 4 328kJ/mol Li + (g) + F - (g) LiF(s) Δ H 5 = Li(s) + 1 2 F 2 (g) LiF(s) Δ H f 594.1kJ/mol Hess’s law: Δ H f = Δ H 1 + Δ H 2 + Δ H 3 + Δ H 4 + Δ H 5 GChem I 9.3
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- 594.1kJ/mol = 155.2kJ/mol + 75.3kJ/mol + 520kJ/mol + ( - 328kJ/mol) + Δ H 5 =⇒ Δ H 5 =- 1017kJ/mol lattice energy of LiF = 1017kJ/mol Compound Lattice energy (kJ/mol) Melting Point ( C) LiF 1017 845 LiCl 828 610 LiBr 787 550 LiI 732 450 NaCl 788 801 NaBr 736 750 NaI 686 662 KCl 699 772 KBr 689 735 KI 632 680 MgCl 2 2527 714 Na 2 O 2570 Sub (1275) MgO 3890 2800 GChem I 9.4
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sharing of one or more pair(s) of electrons between atoms = covalent bond transfer (ionic bond) or sharing (covalent bond) -→ each atom achieves an especially stable electron configuration, often a no- ble gas configuration ns 2 np 6 : octet Lewis symbol: chemical symbol (=nucleus + core electrons) + dots for valence electrons example: Si [Ne]3 s 2 3 p 2 Si · · · · Lewis symbols mostly for main-group elements Lewis structure : combination of Lewis symbols to show cova- lent bonds H · + Cl · ·· · · · · H Cl · · · · · · shared pair = covalent bond bonding pair : lone pair: Cl H . . . . . . . . . . . . . . . . . .
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